MB-37898: Use std::shared_ptr use_count instead of unique
MB-37898: Use std::shared_ptr use_count instead of unique

std::shared_ptr::unique was deprecated in C++17 which generates a
warning. Swap to using use_count instead.

Change-Id: I857b5be109cfe140f5606252fe26fa3f2b4e277d
Reviewed-on: http://review.couchbase.org/122512
Reviewed-by: Jim Walker <1cd02e31b43620d7c664e038ca42a060d61727b9@couchbase.com>
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>

daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine